Sourdough is fermented dough. You can use a sourdough starter that you have nurtured carefully for weeks, or you can just do it this way:

PPut a cup of Bread flour in a large bowl.
Add a teasspoonful of salt
Add a package of Active Dry Yeast
Add a cup of water

Mix thoroughlly, covern and set aside for three (3) days.

Then add another cup of Bread flour - and maybe a llittle more.

Mix until a ball of dough forms and pulls away from the bowl. The dough should be soft but not sticky (add more flour if necessary)

Remove from bowl and form a ball. Clean and oil the bowl, and put the dough back in it, Rotate the dough ball to coat it with oil. Cover and put in a warm place to rise until a deep finger-hole stays open.

REmove to a floured surface, form into a long sylinder, pat into a long, narrow rectangle. Roll into a cylinder from the long side,. Gently place it on a corn meal sprinkled baking sheet. Cover and let rise again until a finger dent doesn't spring back.